There has been an approach to decrease rolling resistance of a tire to suppress the heat build-up and thus improve fuel economy of a vehicle. The demand for improving fuel economy by means of tire improvement has increased in recent years. With respect to a sidewall, an insulation, and a breaker cushion as well as a tread which occupies a large part of a tire, the improvement in fuel economy has been desired. Examples of known methods for achieving low heat build-up of rubber compositions for these tire components include a method using a low-reinforcing filler, and a method using a smaller amount of a reinforcing filler. Also, an attempt has been made to achieve low heat build-up by using silica as a filler.
However, such methods for improving fuel economy using a filler cause reduction in the effect of reinforcing the rubber composition, and thereby problematically cause reduction in flex crack growth resistance. Hence, it has been difficult to achieve both excellent fuel economy and high flex crack growth resistance.
Sidewalls, insulations, and breaker cushions have been commonly produced using natural rubber. Here, natural rubber has a higher Mooney viscosity than those of other synthetic rubbers and thus has low processability. Therefore, natural rubber to be used is usually added with a peptizer and then masticated so that the rubber has a decreased Mooney viscosity. Requirement of such a process in the case of using natural rubber decreases productivity. Further, the mastication causes molecular chain scission in natural rubber, thereby problematically leading to a loss of the properties of a high-molecular-weight polymer that natural rubber essentially has (for example, high abrasion resistance, fuel economy, and rubber strength).
Patent Document 1 discloses a rubber composition prepared using natural rubber and epoxidized natural rubber in order to increase the content of non-petroleum resources. However, the rubber composition still has room for improving both fuel economy and flex crack growth resistance while simultaneously improving processability.    Patent Document 1: JP 2007-169431 A